Cathodic electrodeposition as a coating application method for metallic substrates is well known and discribed for example in U.S. Pat. Nos. 4,575,523; 4,661,541; 4,780,524 and 4,920,162.
The electrocoating composition comprises the cationic resin (principal resin), a crosslinker, a grind resin, pigments and other additives such as solvents, control agents, fillers and the like.
Typically, a principal resin is prepared by adducting an epoxy resin with an amine. An aqueous electrodeposition coating bath is prepared by mixing the principal resin with a crosslinking agent and salting it with acid and deionized water to form a dispersion, mixing the dispersion with a pigment paste and optionally with other additives like solvents, antifoam and the like.
Pigment pastes are usually prepared by dispersing a pigment in a grinding resin in the presence of plasticizers, wetting agents, surfactants or other ingredients in a ball mill, sand mill, cowles mill or continuous mill until the pigment has been reduced to the desired particle size and is wetted by the resin or dispersed in it.
One disadvantage of pigment pastes is that they contain volatile organic compounds.
It is therefore an object of the present invention to provide a process for the preparation of a cathodic electrocoating composition with very low VOC but good pigment dispersibility comprising a powder of a dispersion of a pigment in a solid resin. Another object of the invention is an aqueous cathodic electrocoating composition. Another object of the invention is a process for the preparation of an aqueous electrocoating composition from a one component electrocoating composition.